1. Field of the Invention
The present invention relates to a scanning optical system and image forming apparatus using the system. In particular, it relates to a scanning optical system which uses a polygon mirror as a light deflector to deflect/reflect a light beam emitted by a light source means and to optically scan the light beam on a scanned surface through an imaging optical system, thereby recording image information. The system is suitably used for an apparatus such as a laser beam printer or digital copying machine having an electrophotographic process. Also the present invention relates to a scanning optical system which properly corrects the curvature of field in the main scanning and sub-scanning directions to always obtain high-quality images without any pitch irregularity when the system uses an over-field optical system in which the width of a light beam incident on a polygon mirror in the main scanning direction is larger than the width of a deflection surface of the polygon mirror in the rotational direction, and an image forming apparatus using the system.
2. Related Background Art
As a conventional scanning optical system, an under-field optical system is generally used. In this optical system, the width (to be also referred to as the facet width of the polygon mirror hereinafter) of a deflection surface 95a (deflection/reflection surface) of a polygon mirror 95 as a light deflector in the rotational direction is larger than the width (to be also referred to as the main scanning light beam width of an incident light beam hereinafter) of a light beam incident on the deflection surface 95a of the polygon mirror 95 in a direction corresponding to the main scanning direction. In this scanning optical system, the width of the deflection surface 95a of the polygon mirror 95 is set to totally reflect a light beam incident from a predetermined direction at any angle.
Referring to FIG. 1, a light beam optically modulated and emitted by a light source means 91 is converted into a substantially collimated light beam by a collimator lens 92. The light beam (light amount) is limited by an aperture stop (slit) 93 and incident on a cylindrical lens 94 having a predetermined refracting power only within a sub-scanning cross-section. Of the substantially collimated light beam incident on the cylindrical lens 94, the light beam in a main scanning cross-section emerges in the substantially collimated state. The light beam in a sub-scanning cross-section converges to form an almost line image on the deflection surface (deflection/reflection surface) 95a of the polygon mirror 95. The light beam deflected/reflected by the light deflector is guided onto a scanned surface (photosensitive drum surface) 97 through an imaging optical system 96 having f-xcex8 characteristics. The polygon mirror 95 is then rotated in the direction indicated by an arrow A in FIG. 1 to scan the light beam on the scanned surface 97 in the direction indicated by an arrow B in FIG. 1, thereby recording image information.
Recently, demands have arisen for increases in speed and resolution in image forming apparatuses such as laser beam printers and digital copying machines. In order to meet these demands, in the above under-field optical system, the rotational speed of the polygon mirror may be increased to shorten the time required for a light beam to scan one line on the photosensitive drum surface.
The number of revolutions of a driving motor for rotating/driving the polygon mirror is currently limited to 25,000 to 30,000 rpm with the use of ball bearings, and 35,000 to 40,000 with the use of fluid bearings such as air bearings, which lead to a great increase in cost. There is therefore a limit to the attempt to attain increases in the speed and resolution of an image forming apparatus by increasing the number of revolutions of the polygon mirror.
In order to increase the speed and resolution of an image forming apparatus, the number of times of scanning of the polygon mirror per rotation may be increased by increasing the number of deflection surfaces of the polygon mirror. In the under-field optical system, however, in order to obtain the same spot diameter, the polygon mirror needs to have deflection surfaces each having a size large enough to cover light beams having the same width and reflect them. For this reason, as the number of surfaces of the polygon mirror increases, the diameter of the polygon mirror increases, resulting in a decrease in the number of revolutions of a general driving motor. This makes it difficult to attain increases in speed and resolution.
Under the circumstances, a scanning optical system using an over-field optical system is disclosed in Japanese Laid-Open Patent Application No. 8-171069. In the over-field optical system, an incident light beam whose light beam width in a direction corresponding to the main scanning direction is larger than the width of a deflection surface of the polygon mirror in the rotational direction strikes the polygon mirror, and only the light beam deflected/reflected by a predetermined deflection surface is scanned on a scanned surface. According to this over-field optical system, even if the number of deflection surfaces increases, an increase in the diameter of the polygon mirror can be prevented. This makes it possible to increase the number of times of scanning per rotation without decreasing the number of revolutions.
To obtain a high-resolution image, the spot diameter on a photosensitive drum surface must be reduced to a small value. A spot diameter Dsp is determined as follows by a truncation factor k, f-xcex8 coefficient f, aperture stop size Dap, and wavelength xcex of a light beam to be used according to the following equation:
Dsp=kxc3x97f/Dapxc3x97xcex
In the over-field optical system, since the facet width of the polygon mirror serves as a stop in the main scanning direction, the spot diameter on the scanned surface can be reduced without increasing the size of the polygon mirror. The over-field optical system is therefore very advantageous in increasing the speed and resolution of an image forming apparatus.
In an attempt to increase the resolution, the spot diameter on the scanned surface is reduced. With a reduction in spot diameter, the depth of focus (image plane depth) decreases. It is therefore preferable that curvature of field is properly corrected.
A light beam deflected/scanned by the polygon mirror forms a spot on the scanned surface through an imaging lens (imaging optical system) having f-xcex8 characteristics and is scanned at a uniform velocity. For this reason, distortion (f-xcex8 characteristics) must be sufficiently corrected.
The optical scanning system disclosed in Japanese Laid-Open Patent Application No. 8-171069 is an optical system made up of two cylindrical lenses each having a predetermined power in the main scanning direction and one cylindrical mirror having a predetermined power in the sub-scanning direction. In this optical system, asymmetry such as curvature of field or distortion due to oblique incidence of a light beam from a main scanning cross-section is not sufficiently corrected.
In addition, a deflection surface moves in the direction corresponding to the main scanning direction upon rotation of the polygon mirror. In the over-field optical system in which an incident light beam having a main scanning light beam width larger than the facet width of the polygon mirror strikes the polygon mirror, this movement of the deflection surface appears as a phenomenon of movement of the pupil position. This movement of the pupil position makes it difficult to correct curvature of field, and more specifically, distortion.
Assume that a light beam is incident from a deflecting/scanning surface at a predetermined angle. In this case as well, when a cylindrical lens having a predetermined power in the sub-scanning direction is disposed near the scanned surface, a return angle that can correct the curvature of field in the sub-scanning direction is uniquely determined. That is, there is no degree of freedom in positioning. In addition, the use of an elongated glass mirror leads to a great demerit in terms of cost, e.g., process cost and vapor deposition cost.
A scanning optical system in which an imaging optical system having f-xcex8 characteristics is partly formed by using a reflection optical element may be used. The surface shape of the reflection optical element, however, demands a precision about four times that of the lens surface of a transmission optical element. When a mirror having a power is to be formed by molding using a plastic material, it is difficult to obtain stable molding characteristics. Hence, curvature of field, distortion, and the like may occur. In addition, aluminum or the like must be deposited on the reflection surface, resulting in a great demerit in terms of cost.
It is an object of the present invention to provide a scanning optical system using an over-field optical system and having a simple arrangement, in which each of a plurality of lenses constituting a second optical system, in particular, is properly shaped to properly correct curvature of field and distortion (f-xcex8 characteristics) even if the pupil position moves in the main scanning direction upon rotation of a polygon mirror, and an image forming apparatus which can always form high-quality images without any pitch irregularity.
A scanning optical system of the present invention in which a first optical system makes a light beam emitted by a light source strike a deflection surface of a light deflector with the light beam having a width larger than the width of the deflection surface in a main scanning direction, and a second optical system forms an image of the light beam deflected/reflected by the light deflector in the form of a spot on a surface to be scanned (scanned surface) is characterized in that the second optical system has a plurality of lenses, and at least one of the lenses has a surface whose generating-line corresponding to the main scanning direction is non-arcuated.
In addition, this system is characterized in that the light beam emitted by the light source is incident on the deflection surface of the light deflector in the sub-scanning cross-section at a predetermined angle and is incident on the deflection surface from substantially the center of the deflection angle of the light deflector in a main scanning cross-section.
In addition, this system is characterized in that the light beam emitted by the light source means is incident on the deflection surface of the light deflector in the sub-scanning cross-section at a predetermined angle and is incident on the deflection surface from substantially the center of the deflection angle of the light deflector in a main scanning cross-section,
the imaging magnification of the second optical system is not more than 1,
the lens having the surface whose generating-line is non-arcuated is located closer to the scanned surface than the remaining lenses,
at least one of the lenses constituting the second optical system also serves as part of the first optical system, and
at least one of the lenses constituting the second optical system has one peripheral portion partly cut off.
Furthermore, a scanning optical system of the present invention in which a first optical system makes a light beam emitted by a light source strike a deflection surface of a light deflector with the light beam having a width larger than the width of the deflection surface in a main scanning direction, and a second optical system forms an image of the light beam deflected/reflected by the light deflector in the form of a spot on a surface to be scanned (scanned surface) is characterized in that the second optical system has a plurality of lenses, and at least one of the lenses changes in radius of curvature in the sub-scanning direction along the main scanning direction without any correlation to the generating-line corresponding to the main scanning direction.
This system is further characterized in that the light beam emitted by the light source is incident on the deflection surface of the light deflector in a sub-scanning cross-section at a predetermined angle and is incident on the deflection surface from substantially the center of the deflection angle of the light deflector in a main scanning cross-section,
the imaging magnification of the second optical system is not more than 1,
the lens whose radius of curvature in the sub-scanning direction changes along the main scanning direction without any correlation to the generating-line corresponding to the main scanning direction is located closer to the scanned surface than the remaining lenses,
at least one of the lenses constituting the second optical system also serves as part of the first optical system, and
at least one of the lenses constituting the second optical system has one peripheral portion partly cut off.
Moreover, a scanning optical system of the present invention in which a first optical system makes a light beam emitted by a light source strike a deflection surface of a light deflector with the light beam having a width larger than the width of the deflection surface in a main scanning direction, and a second optical system forms an image of the light beam deflected/reflected by the light deflector in the form of a spot on a surface to be scanned (scanned surface) is characterized in that the second optical system has a plurality of lenses, and at least one of the lenses has a surface whose generating-line corresponding to the main scanning direction is non-arcuated, and changes in radius of curvature in a sub-scanning direction along the main scanning direction without any correlation to a generating-line corresponding to the main scanning direction.
This system is further characterized in that the light beam emitted by the light source is incident on the deflection surface of the light deflector in a sub-scanning cross-section at a predetermined angle and is incident on the deflection surface from substantially the center of a deflection angle of the light deflector in a main scanning cross-section,
the imaging magnification of the second optical system is not more than 1,
the lens having the surface whose generating-line corresponding to the main scanning direction is non-arcuated and changing in radius of curvature in the sub-scanning direction along the main scanning direction without any correlation to the generating-line corresponding to the main scanning direction is located closer to the scanned surface than the remaining lenses.
An image forming apparatus of the present invention is characterized by forming images using any one of the scanning optical systems described above.